You will note of course that their legs are significantly different to ours. They're much shorter, they're less optimally-shaped for full upright standing (when do you ever see monkeys, apes, gorillas, etc standing tall like a human?), and their feet are differently-shaped. I think the comparison is flawed.
There is a book somewhere that talks about this in detail, its not just animals, but in third world countries, people don't sit with their legs and back at ~90 degree angles. Chairs are actually the most unnatural position for our bodies as a sedentary position (especially in the case of toilets while defecating). There are a lot of back and other problems that come with prolonged use of chairs that simply don't occur in areas of the world where people squat or don't have chairs.
Many cultures like the Chinese and Korean still squat a lot instead of sitting, it's funny for us to see but they seem to be enjoying it.
If I stay in a squat position within a couple of minutes my legs become really tensed up and it feels as if my I'm cutting my blood flow; squatting for even 5 minutes is not feasible for me, let alone 8 hours, and I don't believe it to be a matter of training. I think we are a long way from our fellow primates.
In a squatting position, your feet should be flat on the ground (both heels and toes) and your upper legs should be touching your lower legs. Are you sure that your form is correct?
You may not believe it is a matter of training, but your experience is fairly normal for someone who lacks the required flexibility from years of not squatting much.. As long as your form is good, squatting more will quickly rectify that.
Its pretty much how 'sitting' worked before the advent & ubiquity of chairs